As known, the expansion anchors of the above-described type are integrally made of a homogenous plastic material, which is characterized by a particularly low rigidity so as to ensure sufficient deformability and flexibility to the tubular element.
Although on one hand, the anchor having a plastic material characterized by a low rigidity ensures a good radial expansion and an effective plastic deformation capable of conveniently allowing a good anchoring of the anchor itself into walls made of compact materials and/or walls having internal voids, on the other hand such a mechanical feature may prevent a correct, strong coupling of the screw onto the tubular element of the anchor.
Indeed, it often occurs that the progressive screwing of the screw into the longitudinal through opening of the anchor instead of causing a correct helical incision, i.e. a correct self-threading of the screw onto the internal wall of the anchor opening, determines a surface abrasion on the internal wall itself, particularly stressed on the head portion, thus causing an increase and damage of the internal section of the opening itself.
Therefore, in this case, in addition to becoming progressively uncoupled from the anchor, the screw is no longer capable of correctly and completely retracting the head portion of the tubular element towards its collar, thus causing an incomplete, ineffective anchoring of the anchor into the wall.
Furthermore, such an inefficiency is particularly highlighted when the anchor is fixed onto walls made of materials with low compactness and/or having an internal structure presenting voids. Indeed, in these circumstances, the retraction caused by the screw on the head of the anchor determines a deformation of the anchor inside the wall which originates an oversized expanded plastic knotting along the longitudinal axis. Such an oversized expanded plastic knotting serves the function of exerting a tightening torque on the screw so as to anchor it to the wall. However, the low rigidity of the plastic material of which the anchor is made often determines a tightening torque by the plastic knotting on the screw, which may be insufficient to ensure a correct anchoring of the anchor to the wall.
Finally, in the expansion anchors made of a homogenous plastic material of the above-described type, the user can only partially perceive the complete, effective anchoring of the anchor to the wall.